what do these options affect?
The viewport display only displays in the 3D viewport. The surface and volume panels actually provide shortcuts to edit the world nodes, which you can also examine in the shader editor, when you set it to World.
Surface
The surface color of the world when rendering. This color will be visible in the background when rendering and will indirectly light objects. You can see the indirect lighting on the left face of the cube. To enter rendered mode, press Z in the 3D Viewport and choose Rendered.

Viewport Display
The surface color of the world when using the workbench renderer. This color will be visible in the background when not in rendered mode and will not indirectly light objects. You will only see this color in the background in solid or wireframe mode.
You will also have to enable its display in the Viewport Shading settings.
To enter solid mode, press Z in the 3D Viewport and choose Solid.

Volume
You have select a Background BSDF in the Surface Panel which gets connected to the world output Surface. You will have to select a fitting Volumetric BSDF in the volume panel, since it will get connected to the volume output. A volume scatter BSDF scatters light volumetrically.